1. Field of the Invention
This invention relates to fuel injection valves, and more particulary to a fuel injection; valve for a high-pressure injection.
2. Description of the Prior Art
An injection valve of the type with which this invention is concerned has been disclosed in European patent application 0 661 442 A1. Fuel injection valves of this kind have a control chamber which, by means of an inlet throttle bore, continuously communicates with a high-pressure fuel source by way of a high-pressure line. A valve closing member of the fuel injection valve is kept in the closed position as long as the control pressure prevailing in the control chamber is high.
The control chamber can be discharged by means of an outlet throttle bore which is acted upon by an injection control valve. As soon as the injection control valve opens the outlet throttle bore, the control chamber is discharged and the valve closing member of the fuel injection valve switches into its open position so that the injection into a combustion chamber of an internal combustion engine can take place. When the injection control valve closes the outlet throttle bore again, the valve closing member is brought back into the closed position as a result of the pressure increase in the control chamber.
Speed, precision, and reproducibility of the opening and closing movements of the injection control valve are of crucial importance for the quality of the fuel injection. The reproducibility of the opening and closing movements is decisively determined by the design of the injection control valve that is essentially a valve seat that in order to open and close the outlet throttle bore, cooperates with a valve ball that is pressed against the valve seat by a guide piece in order to close and open the injection control valve or is subjected to an initial stress of a spring in order to open it.
A recess in the guide piece that guides the valve ball is in fact adapted to the diameter of the valve ball, but radial displacements of the ball in relation to the ball seat can occur if the high-pressure jet at the outlet throttle bore strikes the valve ball in a radially offset manner. Furthermore transient effects can occur until the ball is lifted up centrally from the valve seat and finally, the embodiment of the valve seat as a flat cone does not assure that the valve ball will close the valve seat without radial displacement and without the occurrence of transient effects during the closing process.
FIG. 3 shows a detail of a typical embodiment of the essential constructive parts of an injection control valve. By means of the crimped screw connection 11, the fuel injection valve is connected to the central high-pressure line 6 which in turn communicates with a high-pressure fuel source. By means of an inlet throttle bore 10, a control chamber 7 is placed under high-pressure which acts on a valve closing member 12 that keeps the fuel injection valve closed as long as the high pressure prevails in the high-pressure control chamber. By means of a discharge bore that transitions into an outlet throttle bore 8, the control chamber 7 can be discharged so that the valve closing member opens the fuel injection valve and fuel from the central high-pressure line 6 is injected into the combustion chambers of an internal combustion engine. The opening and closing of the outlet throttle bore 8 is assured by means of an injection control valve that has a valve seat 2, a valve ball 3, and a guide piece 4 that guides the valve ball 3. The flat cone-shaped valve seat with an obtuse opening angle xcex1 can also be clearly seen here, which has also been disclosed by the reference EP 0 661 442 A1 in FIG. 2.
Each occurrence of transient effects and/or of radial displacements of the valve ball in relation to the center of the centrally disposed outlet throttle bore reduces the precision and reproducibility of the opening and closing movements of the injection control valve.
The object of the invention, therefore, is to overcome the disadvantages of fuel injection valves of the prior art, to assure a reliable, uniform closing of the valve ball in the injection control valve, and to reduce distortions due to transient effects or other obstructions to the valve ball during the closing of the injection control valve.
The disposition of a diffuser between the valve seat and the outlet throttle bore advantageously achieves the fact that compared to the embodiment according to EP 0 661 442 A1, a higher percentage of the kinetic energy of the high-pressure jet emerging from the throttle bore is converted into static pressure. Consequently, due to the greater average diameter of the diffuser in relation to the throttle bore, the pressure contacts a greater service area of the valve ball as it is opening. Consequently, the valve ball is uniformly and reproducibly centered as it is lifting up and radial displacements of the valve ball in relation to the outlet throttle bore can be prevented to the greatest extent possible.
The embodiment approximately in the shape of a steep-walled funnel composed of the outlet throttle bore, diffuser, and valve seat, in which the funnel shape has a right-angled to acute-angled cone angle a, advantageously achieves the fact that in contrast to the conventional valve seat composed of a flat cone with an outlet throttle bore disposed centrally at the tip of the cone, the funnel wall of the valve seat encourages the centering of the valve ball during the closing of the injection control valve and prevents a radial displacement of the valve ball in relation to the diffuser and the outlet throttle bore. Consequently, the embodiment according to the invention of the injection control valve in the fuel injection valve achieves an increased precision and reproducibility of the opening and closing movement.
Usually, the diffuser is a constant widening from a minimal diameter to a maximal diameter. In this connection, increasingly and steadily, the kinetic energy of a flowing medium is partially converted into static pressure. In a preferred embodiment of the invention, the diffuser is embodied as a xe2x80x9ccross sectional jumpxe2x80x9d, i.e. the minimal and maximal diameter of the diffuser are equal. This represents an inconstant widening of the outlet throttle bore to the diameter of the diffuser, which is normally referred to as a Carnot opening. A Carnot opening of this kind has the advantage that the resistance coefficient xc3xa6 can be optimized by means of simply changing the ratio between the diameter of the diffuser and the diameter of the outlet throttle bore.
In a preferred embodiment of the invention, the ratio between the average diameter of the diffuser and the diameter of the outlet throttle bore lies between 1.2 and 2 so that the resistance coefficient xc3xa6 can be set between about 0.16 and about 9.
In another preferred embodiment of the invention, the cone angle xc3xa6 is 60xc2x0 to 90xc2x0. In contrast to the flat cone known from the prior art, this steep-walled cone permits an improved centering of the valve ball. With cone angles of greater than 60xc2x0, the centering of the valve ball is in fact more strongly encouraged, but the ball cannot protrude into the diffuser far enough to be suspended as close as possible to the outlet throttle bore when the injection control valve is closed. On the other hand, with cone anglesxcex1 of greater than 90xc2x0, the centering action of the funnel shape becomes increasingly less effective so that there is an increase in the disadvantages explained above for the prior art.
Preferably, the valve ball protrudes with between ⅕ and {fraction (1/10)} of its radius r into the diffuser. This can advantageously result in the fact that on the one hand, a sufficiently large spherical cap of the valve ball is struck by the high-pressure jet and lifted up from the valve seat in a centered fashion and on the other hand, the valve ball is prevented from protruding too far into the diffuser.
In another embodiment of the invention, the maximal diameter D of the diffuser and the length 1 of the diffuser are matched to one another in such a way that when the injection valve is closed, the valve ball is positioned at a distance of xe2x89xa60.1 mm, preferably between 30 and 80 xcexcm, above the outlet throttle bore. This spacing preferably assures that during the opening of the injection control valve, not only does the high-pressure jet from the outlet throttle bore initially act on the valve ball surface in the vicinity of the throttle bore, but also the pressure acts on the larger surface area of a spherical cap of the valve ball in the vicinity of the maximal diameter of the diffuser or of the valve seat.
The length-to-diameter ratio of the outlet throttle bore is crucial for the percentage of the throttling action. The smaller the diameter and the greater the length of the throttle bore, the more intense the throttling action is. Increasing throttling action also results in a reduced consumption of the fuel emerging from the control chamber. At the same time, however, the time required to decrease the high pressure in the control chamber increases. Therefore, the range from 1 to 20 for the length-to-diameter ratio of the outlet throttle bore represents an optimal compromise between these two extremes.
Furthermore, the diffuser preferably has a length-to-maximal diameter ratio between 0.1 and 0.5. This length-to-maximal diameter ratio of the diffuser results in the fact that the flow does not come into contact with the casing-shaped wall of the diffuser so that the frictional losses in the diffuser become negligibly small while the flow losses increase due to turbulence generation at the step-shaped transition.